The point is that in order to create a competitive app store the notional competitor would have to create an entire hardware ecosystem to rival Apple. Apple is tying the sale of products together (their phone hardware with the app store software) in order to profit from their app store and they prohibit the app stores of competitors. This is the definition of anti-competitive monopoly[1].

You also write that Apple is dominant because it is good and good because it is locked down. This is an argument that's first impossible to verify and second irrelevant. We can't know if "locking down" and preventing other app stores from existing makes Apple better or worse since we can't look at the counter-factual reality where Apple isn't locked down. My intuition would be that more choices would be better, but again, we can't know. The argument is irrelevant because it's still anti-competitive behavior and illegal regardless of if it makes the product better or not. I might sell more soda if I included a bit of cocaine in every bottle, but I couldn't argue that it was legal to do so because it made the product better. Bundling IE with Windows for free may have made Windows better, and yet...

1 - https://www.ftc.gov/tips-advice/competition-guidance/guide-a...

> this is the definition of anti-competitive monopoly[1]

Keep reading your own link.

https://www.ftc.gov/tips-advice/competition-guidance/guide-a...

> Courts look at the firm's market share, but typically do not find monopoly power if the firm (or a group of firms acting in concert) has less than 50 percent of the sales of a particular product or service within a certain geographic area.

Apple has less than a 50% market share on smart phones.

Apple has more than 50% market share in the US[1]. The FTC is meant to protect American consumers, so the fact that Apple has a majority of market share in the US and is abusing their monopoly position in the US is relevant. The fact that Android leads iOS worldwide is less relevant.

1 - https://gs.statcounter.com/os-market-share/mobile/north-amer...

Apple and Google dominate the mobile OS market with iOS being on 60% of devices in the US, and 40% with Android. Apple is responsible for 100% more sales in the mobile app distribution market compared to Google, but together they're responsible for over 99% of all mobile app sales in the US.

Read the definition you quoted from the FTC. Apple and Google are a group of two firms with monopoly positions in the mobile OS market and the mobile app distribution market. Google is even forcing developers to use their payment service on the Play Store and to give them a 30% cut[1] of all app sales to match Apple.

[1] https://www.cnbc.com/2020/09/28/google-to-enforce-30percent-...